.338 Lapua Magnum
Date: Apr 16 2020
The .338 Lapua Magnum dates back to 1983 when U.S. based Research Armament Industries necked the .416 Rigby case down to .338 inch (along with shortening the case and changing the shoulder) in an effort to achieve military adoption. The case, however, was too weak to withstand high pressures of the new wildcat cartridge that would push a 250-grain bullet to around 3,000 fps when loaded to pressures beyond 60,000 psi. Next, RAI had Brass Extrusion Labs Limited make cases, but they too were unsatisfactory. By 1984 Lapua of Finland was contacted, but due to financial difficulties the project was discontinued. In 1985 Lapua went back to work on the project, incorporating small changes when compared to the original .338/416 RAI version, including increasing the strength of the web and sidewalls just forward of the head. Production ammunition was shipping by 1987.
Most of the world’s major militaries have adopted this cartridge, including the U.S., to fill the gap between the .50 BMG and 7.62x51 NATO (.308 Winchester). It has proven an outstanding longrange sniper cartridge, while civilians are finding favor with it in long-range target competitions and hunting.
All major U.S. ammunition manufacturers offer factory loads; however, it has not been officially adopted by SAAMI. It has CIP standards, and most companies are holding maximum average pressures to around 61,000 psi.
Most .338 Lapua rifles are fitted with one turn-in-10-inch rifling, while Sako produces a 12-inch twist. These will properly stabilize all military loads, including various 250-grain bullets. The trend has been toward heavier bullets, up to 300-grains; however, for extreme long-range work. As a result, the Savage Model 110 BA used to develop the accompanying data features a 9-inch twist and offers proper stabilization even when long, heavy bullets are fired. Faster twist barrels will show a slight increase in chamber pressure.
Not all cases handled pressures equally. For example, Lapua has changed case zinc content due to delaminating discovered in new, unfired ammunition. The zinc content was originally high to increase case strength. Some case lot numbers showed sticky extraction and high pressure, while others from the same manufacturer handled the same load with ease. Generally, Lapua cases were of high quality, but Nosler Custom and HSM cases were also used with excellent results.
Most factory loads produced between 2,900 to 2,950 fps with 250-grain bullets. It was easy to duplicate or exceed all factory load ballistics.
To achieve proper powder ignition, a large rifle magnum primer is required, with Federal 215 being used herein.
Top performing (accuracy) powders with 225- and 250-grain bullets included Alliant Reloder 19 and Hodgdon 4831sc. However, several other powders gave almost equal results. Moving up to the 285- grain Hornady BTHP Match bullet, Winchester Supreme 780 gave the highest velocity (over 2,800 fps), while Alliant Reloder 22 and Hodgdon Retumbo showed a slight accuracy edge.